Difficulties are often encountered in electrical circuits because of the variation in line voltage and errors due to the tolerance levels of the various components. This is particularly true in phase fired control circuits when there are large line voltage variations and many components involved. Variations caused by these factors lead to large shifts in the firing angle of the control circuit. Additionally, the setting of a control resistor can present problems if setting of the optimum operating point is required at nominal line voltage at the center of the variable control resistor travel.